1. Field of the Invention
The present invention is directed to a method and apparatus for detecting a sequence of abnormal events among a plurality of normal events in an electrical signal. The method and apparatus are particularly suited for detecting a sequence of events indicating fibrillation among a plurality of normal events in the depolarization signal of a heart, wherein the fibrillation is detected and defibrillation therapy is automatically administered.
2. Description of the Prior Art
The detection of a sequence of abnormal events among a plurality of normal events in an electrical signal is necessary for detecting arrhythmias in the depolarization signal of a pathological heart. In addition to waveforms indicating normal heart activity, the depolarization signal of a pathological heart contains waveforms which occur given the appearance of an arrhythmia, for example fibrillation, which deviate from the waveforms which are present in the case of normal heart activity. If the waveforms corresponding to a normal heartbeat are considered to represent a normal event in the heart depolarization signal, and the waveforms corresponding to an abnormal heartbeat are considered to represent an abnormal event in the depolarization signal, an arrhythmia represents a sequence of abnormal events among a plurality of normal events in the signal. Since a heart arrhythmia may present a life threatening situation, it is desirable to be able to quickly and reliably detect a sequence of events indicating an arrhythmia among a plurality of normal events in the heart depolarization signal to permit initiation of therapeutic measures sufficiently rapidly. Particularly for automatic defibrillators, it is necessary to accurately and reliably detect a sequence of events indicating fibrillation of the heart among a plurality of normal events. Only by doing so is it assured that first, the heart receives the defibrillation pulses required for terminating the fibrillation and second, that unnecessary defibrillation pulses, which can endanger the patient, are avoided.
A device for detecting heart rhythm irregularities, is described in U.S. Pat. No. 3,861,387. In this device, a signal corresponding to the electrical activity of the heart is acquired and the first time derivate of this signal is formed. The peak amplitude of the signal is calculated for each normal or abnormal event in the differentiated signal, and the mean value is continuously formed for a defined plurality of the most recently measured peak amplitudes. Proceeding from this continuously calculated mean value, a threshold is defined and the duration for which the amplitude is above the threshold over the duration of the event is calculated for each event in the differentiated signal. The mean value of a defined plurality of most recently measured time durations is also continuously formed. The peak amplitude, as well as the chronological duration for which the amplitude was above the threshold, are compared to the respective mean values for each event which has occurred in the differentiated signal. As soon as one event exhibits a difference, exceeding a defined limit, form at least one mean value, a signal is generated which indicates the presence of an abnormal event. The decision as to whether an abnormal or a normal event is present is thus not based on objective criteria. To the contrary, it ensues with reference to criteria, namely, mean values, which are defined from the signal being investigated. This may, nonetheless, lead to unusable results in the detection of abnormal events, because events which are detected as abnormal are not taken into consideration in the formation of the mean values. The detection of sequences of abnormal events requires that the generation of signal indicating one abnormal event be suppressed until a defined plurality of abnormal events has been detected within a selected time interval. This is not possible with adequate reliability in the system described in U.S. Pat. No. 3,861,387. This is because if a signal initially contains only objectively normal events and the number of abnormal events occurring within the signal changes only gradually, these events will not be detected because a gradual change of the mean values occurs in gradual change of the signal, and eventually objectively abnormal events will come to be erroneously considered as normal events.